The most common and most likely answer to this crossword clue is the 11 letter word DOCUMENTARY
I've seen this crossword clue in The Guardian
D
O
C
U
M
E
N
T
A
R
Y
More clues leading to the result DOCUMENTARY
Buddy explains!
The straight part
We know that "supposedly truthful programme" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case supposedly truthful programme = documentary.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DOCUMENTARY mean?
DOCUMENTARY noun- A film, TV program, publication etc. which presents a social, political, scientific or historical subject in a factual or informative manner.
- Of, related to, or based on documents.
- Which serves to document (record and:or illustrate) a subject.
- (of a film, book etc) Presented objectively without the insertion of fictional matter.